Sorry, I might have not told exactly want I want... next up:
- 1st of all: I am (almost) completely aware about, how a CDN is working. This is not my problem.
- I am not using a wp or other open source cms. We run a selfdeveloped ecommerce-System.
- We implement a media asset management system to our site, what is using a cdn. The vendor for the cdn system will be akamai. I am not sure about at the moment,
if we can take advantage of the whole akamai feature... - Talking about a "host": I ment the host in termns of the domain. Equivalent would be "subdomain". Say this is our Domain shop.com. In my world "www" is a "host" (subdomain) of the domain shop.com as well as "cdn" would be in your example.
We have thousands of product images indexed by google what brings us lot of traffic and conversions .. we worked hard on that and its working just fine ! 
All of our content (i.e. application, styles, js a.o.) and especially our productimages can be refered under the one and only subdomain "www".
An indexed image might be
www.shop.com/images/my-beautifull-image.jpg
With the implementaion of the cdn, the host "www" has to change, say "cdn", because otherwise the delegation (cname) to the cdn is not possible.
So the image can be refered also by
cdn.shop.com/images/my-beautifull-image.jpg
As mentioned above (and for some technical scenarios) the old imageurl of the product will be valid. So for one image, I will have *two" URLs:
www.shop.com/images/my-beautifull-image.jpg
cdn.shop.com/images/my-beautifull-image.jpg
And my question is: How to handle this in the best way:
- don't let the old subdomain "www" happen, when the new is on... I loose all backlinks to images then and everything has to be reindexed.
- Redirect www -> to cdn, if requested ...
- sending a http-Canoncial tag pointing to the cdn-Host "cdn" when requested on the domain "www".
- exclude the old "www" host in the robots.txt (uuuuuaahhh dont like that)
... or still: not worry about it, Google will take care of it ?? Hmmm...
Thanx anyway
